1. Field of the Invention
This invention is related to the preparation of variable amounts of diphenylamine (DPA) and aniline, and more particularly, is related to the preparation of DPA, using only a phenol-ammonia feedstock, from the resulting reaction of aniline (prepared in situ) with phenol.
2. Description of Prior Art
DPA is a well known industrial chemical having a variety of usages, such as in the production of anti-oxidants for elastomers and in the manufacture of azo dyes, and can be prepared by a variety of methods. Generally, it has been prepared commercially by catalytically de-ammoniating or self-condensing aniline. British Pat. No. 752,859 discloses such a process in which aniline is reacted in the presence of activated alumina or titania. U.S. Pat. No. 3,071,619 discloses a liquid phase process involving the use of NH.sub.4 BF.sub.4 as the catalyst.
Processes for the production of aniline type compounds through the liquid or the vapor phase amination of phenolic compounds with ammonia have also been reported in the literature. Generally, these reactions take place at elevated pressures and temperatures, in the presence of catalysts containing silica and alumina. One such aniline reaction process is disclosed in U.S. Pat. No. 3,272,365, wherein phenol and ammonia are combined over a silica-alumina catalyst at temperatures between 400.degree.-480.degree. C. U.S. Pat. No. 3,860,650 describes a process for the production of organic amines by the amination of phenolic type compounds, using a catalyst consisting essentially of an alumina gel containing less than 1.0 wt. % of alkali metal.
It has been proposed to produce DPA by catalytically reacting aniline with phenol, viz; EQU C.sub.6 H.sub.5 NH.sub.2 +C.sub.6 H.sub.5 OH.fwdarw.C.sub.6 H.sub.5 NHC.sub.6 H.sub.5 +H.sub.2 O
Such a process is described in U.S. Pat. No. 2,824,137 employing a titanium catalyst; in British Pat. No. 1,541,153 using phosphoric acid as a catalyst, and in Japanese Patent Publication No. 75/07.061, which carries out the reaction in the presence of alumina. U.S. Pat. No. 3,944,613 discloses a liquid-phase process for the production of DPA which uses a silica/alumina catalyst. All of the processes described in these publications, however, are characterized by short catalyst lifetime, relatively high temperature operation, and subsequent catalyst separation problems. It has long been recognized that while phenolic type compounds frequently will easily react with aniline, the reaction of aniline with phenol itself is far more difficult to sustain.
U.S. application Ser. No. 431,532, filed Sept. 30, 1982, discloses a process for the production of variable amounts of aniline and DPA from an ammonia and phenol feedstock by using a two reactor process operation. This process essentially features the integration of an aniline plant with a DPA plant, and can achieve desired ratios of aniline/DPA by diverting at least a portion, if not all, of the formed aniline to the DPA plant. In contrast, the present invention can achieve any desired ratio of DPA to aniline using only a single reactor with the attendant savings in plant construction. Also, whereas the cited application discloses a preferred NH.sub.3 /phenol range of 10 to 25 molar, the present application takes cognizance of the effect the NH.sub.3 /phenol ratio has on DPA production and takes advantage of the fact that DPA production is increased at the lower NH.sub.3 ratios.
Catalysts useful for the commercial production of organic amines such as DPA must satisfy certain performance characteristics if they are to prove satisfactory. Catalytic activity, often expressed in terms of the percentage of raw material conversion to the desired product under a particular process environment, is particularly a major concern. The more active a catalyst, the smaller the reaction space, or volume, required for a particular rate of production or, alternatively, the lower is the reaction temperature or flow rate which will be required for the desired production rate. Thus, it is of particular importance to utilize a catalyst which displays sufficient activity at relatively low temperatures, since the lower the operating temperature the greater is the desired selectivity of product, and the longer the catalyst life expectancy.
It is accordingly, an object of this invention to provide an improved process for the preparation of DPA using only a phenol and ammonia feedstock, from the catalytic reaction of aniline (prepared in situ) with phenol in a process having attractive conversions and high selectivities, and yet which avoids the drawbacks and disadvantages of prior art processes.
It is another object of this invention to produce variable amounts of DPA and aniline from a process using only phenol and ammonia as its feedstock, in an efficient and economical manner.
It is still another object of this invention to provide a catalyst useful for the production of both DPA and aniline which has high activity at relatively low reaction temperatures and which is particularly selective to the desired amination reaction.